1. Field of the Invention
The present invention relates to a powder coating composition that is capable of forming a coating film with excellent external appearance and scratch resistance.
2. Description of Related Art
Powder coatings are widely used for the general coating of metals, as environmentally friendly coating materials that do not release organic solvents into the atmosphere during application. Of such coatings, powder coating compositions that include an epoxy group-containing vinyl copolymer as the primary component and use a polyvalent carboxylic acid as a curing agent component enable the formation of coating films with particularly superior levels of gloss, weather resistance, and chemical resistance, and are consequently particularly widely used.
In recent years, considerable research has been undertaken into powder coating compositions for future use as the top clear coatings for automobiles. If environmentally friendly powder coating compositions were able to be used to completely replace the conventional solvent-based coatings used within the enormous market of automobile top clear coatings, then the level of environmental impact would be able to be reduced dramatically. However, in the case of these automobile top clear coatings, the required performance in terms of the external appearance and scratch resistance of the coating film finish is very high, and satisfying these performance levels with conventional powder coating compositions has proven difficult, meaning solvent-based coating materials continue to be used.
In order to meet these needs, for example, a powder coating composition that includes, as the primary component, a silicone-acrylic copolymer in which an organopolysiloxane and an acrylic copolymer are bonded together to form a graft or block copolymer has been reported as exhibiting excellent leveling properties upon melting and excellent scratch resistance (for example, see patent reference 1). However, because this technique employs a large quantity of an organopolysiloxane, problems such as a deterioration in the sharpness of the coating film, and an increase in the likelihood of cissing arise, and the scratch resistance is also unable to satisfy the high levels of performance that are now being demanded, meaning there is still considerable room for improvement.
Furthermore, methods in which two epoxy group-containing acrylic resins are blended together to yield coating films that combine favorable smoothness with favorable scratch resistance have also been proposed (for example, see patent reference 2 and patent reference 3). Because use of these powder coating compositions enables the formation of superior coating films that satisfy the requirements for excellent external appearance and scratch resistance, the use of such compositions within applications such as the formation of automobile top clear coatings, which have conventionally avoided the use of powder coating compositions due to problems associated with the external appearance and scratch resistance performance of the produced coating films, has now become possible, and these powder coating compositions are now starting to be used as replacements for conventional solvent-based coating materials in some areas.
However, with recent moves to more up-market automobile coatings, demands continue to grow for even more improvement in the external appearance of coating film finishes. In addition, demands for higher levels of scratch resistance, which will either prevent or not show scratching when the vehicle is subjected to washing in a car-wash machine, are also growing.
Moreover, from the viewpoints of reducing the quantity used of powder coating compositions, reducing the level of energy consumption, and reducing the overall costs associated with the coating process, demands for reducing the thickness of formed coating films are also considerable. In other words, a composition is required that enables a reduction in the film thickness from the thick films employed with conventional powder coating compositions, and yet also exhibits superior external appearance for the coating film finish and a high level of scratch resistance.
Under these circumstances, even the powder coating compositions described above require further improvement in the level of scratch resistance.
[Patent Reference 1]    Japanese Unexamined Patent Application, First Publication No. Hei 09-241538
[Patent Reference 2]    Japanese Unexamined Patent Application, First Publication No. Hei 09-227799
[Patent Reference 3]    Japanese Unexamined Patent Application, First Publication No. 2003-292881